1. Field of the Invention
The invention relates to precipitated silicas doped with two, three or four-valent metals, to a process for preparing them, and to the use of these doped precipitated silicas.
2. Description of the Background
Silicas and aluminum silicates prepared by precipitation using sodium silicate solution and sulfuric acid, sodium silicate solution and subsequent addition of a soluble metal salt are known. In these preparation processes, metal ions in the form of their salts or solutions thereof—for example, Zr, Ti, Zn ions—are added in a variety of ways. These ions may also enter into chemical bonds with constituents of the silica/silicate surface and may not be removed by simple washing. These ions generate cationic charges on the surface of the silicas/silicates and as a result, when used in the coating of inkjet papers, for example, ensure fixing of the usually anionic dyes and ensure bright colors in the paper coating.
For use in the paper industry there is a need for fillers which in inkjet media, for example, absorb the ink effectively and maintain the brightness of the colors. In order to be able to raise the printing speed and to reduce the size of the printed dot in inkjet printing, rapid drying is vital. One way of meeting these requirements is to apply coatings containing silica to the media. These coatings permit rapid ink absorption, enhance the dot definition, and promote the defined circular propagation of the ink droplet. Furthermore, they prevent showthrough or strikethrough of the ink and produce high color densities.
For use in the paper industry, therefore, there is a need for fillers which are extremely easy to disperse, which absorb the ink well in inkjet paper or inkjet film, for example, and which maintain the brightness of the colors.
The preparation of doped and undoped silicas and silicates has already been extensively described—for example, in EP 0 643 0 15, DE 117 2245 or DE 124 5006.
All of the precipitations described therein for preparing silica comprise three process steps: 1.) introduction of water and, optionally, sodium silicate solution, optionally adjustment of pH, conductivity by adding salts or solutions thereof (e.g., sodium sulfate); 2.) precipitation phase: here, usually by adding a mineral acid such as sulfuric acid, the precipitation of the silica or silicate is brought about; 3.) acidification of the silica/silicate suspension prior to further workup. All three phases are characterized by a specific temperature, metering and pH regime, possible interruption stages and/or intermediate stages, or the addition of different salts or solutions thereof.
In order to produce cationic sites on the surface of the silicas/silicates, metal ions that are at least divalent are added to the precipitated silica (EP 0 493 203). The metals in question may comprise alkaline earth metals, rare earth metals, transition metals (e.g., Ti, Zr, Fe, Ni, Zn), or aluminum. These metals may be added as ions in the form of their salts or solutions thereof. The salts may comprise organic salts or complexes, examples being carbonates, polycarbonates, or else inorganic salts such as halides, oxyhalides, nitrates, phosphates, sulfates, oxide sulfates, hydroxides, oxide hydroxides.
The ions mentioned manifest their activity especially if they are integrated (chemically bonded and/or physically fixed) into the surface of the silicas or silicates. For this to be so, however, the treatment of an already precipitated silica or silicate (suspensions thereof) with salts or solutions of said ions is not sufficient.
EP 0 492 263 discloses such processes. Here, metal salts for doping are applied either to pre-prepared and resuspended silica or to silica suspensions which have already been subjected to precipitation but not yet filtered. In both cases, although the metal ions are deposited on the surface of the particles, there is no chemical incorporation of the metals into the silicate structure. Doped silicas prepared in this way bleed easily, and the metal ions may be released.
It is an object of the present invention, therefore, to provide metal-ion-doped silicas in which the metal ions are to be substantially embedded into the silicate structure.